1. Field of the Invention
The present invention relates to a process for preparing aromatic carbonates by transesterification of aliphatic carbonates with phenolic compounds in the presence of titanium compounds. In this process, before work-up, the transesterification mixture is cooled to a temperature below 120.degree. C., during which the mixture must remain liquid, the titanium-containing precipitate which is deposited is separated off, and the aromatic carbonate is obtained by methods which are conventional per se.
2. Description of the Related Art
It is known that alkyl aryl carbonates and diaryl carbonates can be obtained by transesterification of dialkyl carbonates with phenolic compounds, suitable catalysts being titanium compounds. Such catalysed transesterifications are described for example in German Offenlegungsschrift 2 528 412 and German Offenlegungsschrift 2 552 907. Although titanium compounds per se are very suitable catalysts, the selectivity, particularly at long reaction times and high temperatures, leaves something to be desired, as is confirmed for example in EP 879.
It is therefore expedient not to set the degree of conversion at too high a level, since this would require long times at high temperatures; instead, the aim is to work up the transesterification mixture after achieving relatively low to medium degrees of conversion, as is also described in the aforementioned German Offenlegungsschriften.
Such transesterification mixtures have to be worked up in order to obtain the aromatic carbonates. This can be performed for example by distillation or crystallisation. Crystallisations of such mixtures are however complex operations, particularly since a substantial removal of the catalyst must be ensured. A distillative working-up is therefore preferred on account of this requirement. Since high boiling point compounds are to be separated from one another, the whole reaction mixture that is to be worked up again comes within the range of high temperatures combined with long residence times, resulting in secondary reactions, which is just what the low degrees of transesterification were intended to avoid. If on the other hand efforts are made to reduce the distillation temperatures under a very high vacuum, large gas volumes and correspondingly large apparatus combined with high investment must be expected.
The aforementioned difficulties could be avoided if the catalyst responsible for the secondary reactions could be removed from the transesterification mixture before the working-up.